The capture of high-quality imagery using reflected ambient light involves two fundamental challenges: (i) detection of optical intensities as low as a single photon, and (ii) detection over a wide dynamic range of optical intensities. To detect single photons with high fidelity (i.e., a high signal-to-noise ratio [SNR]), detectors must provide large electrical gain in the optical-to-electrical conversion process. In essentially all semiconductor photodetectors of visible and infrared light, each absorbed photon creates just a single free-carrier pair that includes one free electron and one free hole. Some prior-art technologies can achieve high gains by amplifying each single photo-excited carrier; for instance, image intensifier tubes exploit microchannel plates to obtain 104-106 electrons for each incident photon. However, such large gains often give rise to saturation effects in the presence of larger optical input signals and can severely limit the dynamic range of such imagers.
Other prior-art technologies employ a much lower gain conversion process (often limited to unity gain) and obtain measurable output signals by integrating photo-excited charge for a sufficiently long period of time. These integration strategies often involve the collection of charge on a capacitive circuit element. Thermal fluctuations give rise to noise associated with electrons randomly moving on to and off of integration capacitors, and for practical (i.e., non-cryogenic) operating temperatures, these imagers have sensitivities limited to values well above the single-photon limit. Moreover, the finite size of these integration capacitors establishes a constraint on the dynamic range of the signals that can be recorded during each integration cycle.
The family of semiconductor photodetectors known as avalanche photodiodes (APDs) provides optical-to-electrical gain by exploiting carrier multiplication through the impact ionization process. An APD is designed so that photo-excited carriers induced in the absorption region of the device are injected into a multiplication region where they are accelerated by a large electric field. When an injected carrier reaches a sufficiently high kinetic energy, it can generate another free electron-hole pair through an inelastic collision with lattice atoms in a process referred to as “impact ionization.” These newly liberated carriers are then accelerated, and the process continues to create an “avalanche” of charge until all carriers have exited the high-field multiplication region of the device.
At a sufficiently large electric-field intensity, known as the “avalanche breakdown field,” there is a finite probability that the avalanche multiplication process can lead to a self-sustaining avalanche. By applying a field larger than the breakdown field, the APD is operated in a metastable state in which the injection of a single photo-excited charge can trigger the development of an easily detectable macroscopic pulse of charge in an extremely short (e.g., <1 ns) avalanche build-up period. This so-called “Geiger-mode” operation can provide high-efficiency detection of single photons, and devices operated in this regime are referred to as Geiger-mode avalanche diodes (GmAPDs).
GmAPDs have been used in each pixel of a focal-plane array (FPA) to create 2D passive imagers with single-photon sensitivity. GmAPD-based arrays have also been used for precise time-of-flight measurements in 3D LIDAR imaging.
State of the art GmAPD arrays are typically limited in the pixel formats (128×32, currently). Scaling is possible, but faces significant technology and cost challenges. The driver for scaling the pixel format is to achieve a greater field-of-view (FOV) for the array. The FOV achievable for a GmAPD array is constrained mainly by two factors. One factor is the required angular resolution per pixel, which in turn determines the image resolution per pixel. This is critical, for example, in LIDAR imaging applications wherein object recognition is for the purposes of obstacle avoidance and maneuvering. The other factor is an optical consideration. For GmAPD arrays, the active detection area is typically much smaller than the pixel pitch, which reduces fill factor. This is compensated for by using a micro-lens and then limiting the f-number of the focusing optics, which directly constrains the FOV.
As previously indicated, scaling to larger pixel formats will address these constraints. But presently, such scaling faces significant technological and cost factors.